1. Field of the Disclosure
The present invention relates to a digital power monitoring circuit and system to provide digital power monitoring, including in a switching power supply. More specifically, the digital power monitoring circuit utilizes two delta sigma analog to digital converters (ADC) to provide first and second digital signals containing information about the current through the output inductor and the output voltage, respectively, of the switching power supply and a convolver circuit to provide a third digital signal indicating the power dissipated through the output inductor based on first and second digital signals.
2. Related Art
Generally, digital power monitoring (DPM) involves calculating the power delivered thru an inductor in a switching power supply over an averaging interval. Measuring the power delivered through an inductor can be difficult in switching power supplies, however, information about the power can be used to monitor the condition of the load, to optimize load conditions, to maximize efficiency, and for thermal protection. Thus, it is very useful to monitor this parameter, particularly in a power supply for a microprocessor, for example.
FIG. 1 illustrates a simplified diagram of an exemplary switching power supply 10, specifically a typical buck converter. While FIG. 1 illustrates a buck converter, a variety of switching power supply topologies and architectures share the following common features: switch(s) (Q1, Q2), an output inductor (L) and an output filter capacitor (Cout). In all cases, the voltage across the inductor L is switched between two nodes to control the output voltage (Vout) provided across the output capacitor (Cout). Switching power supplies are popular in a variety of applications and are often chosen for their high efficiency, ability to create a wide variety of voltages, and compactness.
The current in an ideal inductor changes in proportion to the voltage applied across it. That is, in an ideal inductor, the following is true:V=L*di/dt  Equation 1However, the absolute current in an ideal inductor is independent from the voltage across it. In order to have knowledge of the absolute current in the inductor, it is necessary to accurately integrate the voltage applied across the inductor and know the initial conditions, or measure the current with a separate sense element.
Resistor current sensing is a common method of creating a voltage signal which is proportional to the current flowing in an inductor of a switching power supply. As can be seen with reference to FIG. 2, in this method, a resistor (Rsense) is put in series with the output inductor (L) of the switching power supply 10, and generates a voltage (Vcs) across it (following Ohm's Law) which is proportional to the current (Iinductor) in the inductor (L). The current across the resistor Rsense may thus be expressed asVcs−Vout=Iinductor*Rsense  Equation 2The voltage Vcs−Vout is proportional to the inductor current (Iinductor). From this, the following equation can be used to determine the inductor current:Iinductor=(Vcs−Vout)/Rsense  Equation 3
Another method for providing a voltage indicative of a current in an inductor is inductor DC resistance (DCR) current sensing. DCR current sensing typically relies on the copper winding resistance of the inductor (L) which has a significant temperature coefficient. If an ideal inductor were used, the average phase voltage (Vphase) at the switching node would be equal to the output voltage (Vout). However, in realized power supplies there is a small, but measurable difference between the average phase voltage (Vphase) and the output voltage (Vout). This difference is the result of the real intrinsic resistance (DCR) of the inductor L, for example. This difference can be measured by filtering the phase voltage (Vphase) at the switching node and comparing it to the output voltage (Vout). This is described further with reference to FIG. 3. It is noted that the time constant of the chosen for the filter formed by the resistor Rcs and the capacitor Ccs should match the time constant of the inductor (L) and the intrinsic resistance DCR. This will allow Vcs−Vout to be correct even during transients, that is, changes in the switching duty cycle of the power supply 10.
Since the average drop across the inductor L is proportional to the voltage drop across the parasitic winding resistance, the following DCR equations result:Average(Vphase)−Vout=Iinductor*DCR, and  Equation 4Average(Vphase)=Vcs;  Equation 5Thus,Vcs−Vout=Iinductor*DCR.  Equation 6The voltage Vcs−Vout can be viewed as being proportional to the inductor current (Iinductor) such that the inductor current may be determined as follows:Iinductor=Vcs−Vout/DCR.  Equation 7
A useful tool in power supply monitoring/controlling, in general, is a delta sigma type analog to digital converter (ADC) circuit. A delta sigma ADC derives a synchronous (clocked) stream of zeros and ones in which the ratio of ones to zeros, that is, the ones density, contains information regarding the input signal which is digitized. Generally, the ones are counted over a specific conversion interval in a digital filter. The count at the end of the conversion interval is proportional to the signal being digitized. Thus, the average of the signal over the conversion signal is given by the count at the end of the averaging. Since delta sigma digital conversion is generally well known, it is not discussed in further detail herein. However, the output Dx of the delta sigma type ADC circuit may be of particular use in a monitoring circuit since the output Dx is a continuous steam of synchronous (clocked) bits where the ratio of ones (one's density) contains the relevant information regarding the input signal which is digitized. FIG. 4 illustrates an exemplary embodiment of a delta sigma ADC circuit 400.
FIG. 6 illustrates an example of a switching power supply incorporating digital current sensing (DCS) in the form of a modified delta sigma ADC 400′ to facilitate negative inductor currents as well as positive inductor current. Specifically, in FIG. 6 a delta sigma type ADC circuit 400′ is modified to measure both positive and negative output inductor currents. The circuit of FIG. 6 is also described in assignee International Rectifier Corporation's copending U.S. application Ser. No. 12/037,380 filed Feb. 26, 2008, entitled DIGITAL CURRENT SENSE, the entire contents of which are incorporated by reference herein. It is noted that since the feedback loop (including the comparator, flip-flop and the switched current source) holds the inputs to the comparator to be substantially equal, the analysis of the steady state operation of this circuit is simplified as follows:Vcs=Vout  Equation 8Summing the current at the node Vcs, the current flowing through Rcs=average current flowing through the switched current source 600. Since Vcs=Vout we get:Average(Vphase)−Vout/Rcs+Idc=Iswitch*DI  Equation 10When we substitute Vout for Vcs the result is:Iinductor*DCR/Rcs+Idc=Iswitch*DI  Equation 11Since the current Iswitch is defined as Vref/Rref, and Idc is defined as Vref/Rref*K, the result is:Iinductor*DCR/Rcs+(Vref/Rref*K)=Vref/(Rref*K)  Equation 12Solving for DI, the result is:DI=Iinductor*DCR*(Rref/Rcs)*1/Vref)+1/K  Equation 13Thus, DI is a function of the inductor current (Iinductor) and constants. Further, it is noted that Rref is the opposite temperature coefficient of DCR and thus, this formula is temperature independent, which is preferred. In addition, it is noted that DI has an “offset” 1/K in its duty ratio. That is, at zero inductor current, DI will still give a positive duty ratio of 1/K. For example, if K were 2, then at zero current, the duty ration DI would be 50% (50% ones). Solving for the inductor current Iinductor:Iinductor=(DI−1/k)*Vref/DCR)*(Rcs/Rref).  Equation 14It is noted that similar functionality can be accomplished by switching the location of the DC current source 606 and the switch current source 600 (with an inverter required between the output of the D flip flop and the switch current source).
While specific embodiments of a delta sigma ADC circuit have been explored, it is noted that any suitable delta sigma ADC front end may be used to derive the bit stream DI which is proportional to the inductor current.
Thus, there are several ways in which provide a digital signal representing a sensed parameter of the power supply. It would be beneficial to provide a reliable digital power monitoring circuit with low power and a reduced component count.